Magnetic recording media provide excellent advantages which cannot be obtained with other recording systems. Magnetic recording media enable repeated use of media, easily process signals in an electronic form, enable the construction of a system combined with a peripheral equipment, and can modify a signal. Thus, magnetic recording technique has been widely utilized in various fields, such as video equipment, audio equipment, computers and the like. To miniaturize equipment, satisfy the demand for improvement in the quality of recorded and reproduced signals, as well as the demand for prolongation of recordable time and the demand for increase in the recording capacity, it has been desired to further improve the recording density of recording media. For these purposes, improvements in magnetic materials have been made to improve the surface properties of the magnetic layer, the dispersibility of magnetic particles in the magnetic layer and the magnetic properties of the magnetic layer.
Floppy discs having a magnetic layer on a flexible nonmagnetic support, which are used as external recording media for microcomputers and personal computers, require a large capacity as high as 10M bytes or more to manage improved application software and the increase in the amount of data to be processed.
A recording system for high density code having a frequency component region 1.5 times wider than that of conventional codes, such as the RLL signal, has been proposed. Thus, the shortest recordable wavelength of signals to be recorded on these floppy discs approaches 3.0 .mu.m or less, even 1.5 .mu.m or less.
To enhance the recording density of the system, the gap length (.delta.) of magnetic heads has been reduced as well, nearing 0.5 .mu.m or less.
To enable high density recording with a short recording wavelength, it is first necessary to enhance the coercive force of the magnetic layer. To this end, JP-A-58-122623 and JP-A-61-74137 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application") propose that a ferromagnetic metal powder in the form of ferromagnetic particles be incorporated in the magnetic layer on disc-shaped media.
JP-B-62-49656 and JP-A-60-50323 (the term "JP-B" as used herein refers to an "examined Japanese patent publication"), and U.S. Pat. Nos. 4,629,653, 4,666,770 and 4,543,198 disclose use of hexagonal system ferrites, such as barium ferrite, as ferromagnetic particles.
In a magnetic recording medium for computers, such as a floppy disc, overwriting of signals having different recording frequencies is indispensable. Conventional media were satisfactory if they allowed overwriting of two kinds of signals in a double-frequency relationship, i.e., if and 2f signals. However, the above mentioned RLL signal system requires not only the reduction of recordable wavelength, but also overwriting of a plurality of signals having a frequency ratio of 3:8.
If signals having a short recording wavelength and a great difference in recording frequency are used, overwriting of a signal with a shorter recording wavelength on a signal with a longer recording wavelength cannot be successfully performed when the magnetic properties of the magnetic layer are merely improved as disclosed in the above cited JP-A-58-122623 and JP-A-61-74137. In conventional magnetic media, when a signal with a shorter recording wavelength is overwritten on a previously recorded signal with a longer recording wavelength, the magnetic line of force does not reach deep in the magnetic layer, and thus cannot erase the previously recorded signal with a longer wavelength.
To overcome this difficulty, it is most effective to reduce the coercive force of the magnetic layer or reduce the thickness of the magnetic layer. However, the reduction in the coercive force of the magnetic layer causes a reduction in the resolution of recording signals or the reproduced output, making high density recording more difficult.
Another problem encountered with conventional magnetic recording discs is electrification of the media. The occurrence of dropouts resulting from the attachment of dust to the surface of the magnetic layer caused by the electrification of the media must be inhibited. Since these magnetic recording discs are adapted to record computer data in digital form, interruptions in recorded or reproduced signals due to dropout causes an increase in B.E.R. (bit error rate).
The shorter the recording wavelength of signals to be recorded in these magnetic recording discs, the greater the influence of dropout and the deterioration of running durability on the magnetic layer. Thus, this electrification problem is an important consideration in the design of a magnetic recording disc having a large capacity and a high recording density.
An ordinary method for inhibiting the electrification of media is to incorporate carbon black in the magnetic layer. However, even if the thickness of the magnetic layer is reduced, taking into account the problem caused by overwriting in recording of a short recording wavelength region, the amount of carbon black retained in the magnetic layer is limited. Further, to maintain superior the magnetic properties, the incorporation of carbon black in the magnetic layer is preferably avoided.
As another means of solving the electrification problem, a magnetic recording medium has been proposed comprising a nonmagnetic layer containing carbon black, or the like, provided interposed between a magnetic layer and a nonmagnetic support. This magnetic recording medium is disclosed in, e.g., JP-A-55-55432, JP-A-50-104003, JP-A-62-214513, JP-A-62-214514, JP-A-62-231417, and JP-A-63-31027, and U.S. Pat. No. 3,440,091. The magnetic recording medium having such a layer construction is proposed also to improve the surface properties or running durability of the magnetic layer.
However, the magnetic recording media disclosed in the prior art are not sufficiently thin for the recording wavelength of signals to be recorded in large capacity magnetic recording discs which are currently required.
When the thickness of the magnetic layer is reduced to improve the overwritability (i.e., the overwriting property) of magnetic recording discs, the amount of a lubricant which can be retained by the magnetic layer is reduced, deteriorating running durability. After repeated sliding movement with contact the magnetic layer and the magnetic head, lubricant runs short, causing a rise in the friction coefficient. As a result, the magnetic head will scrape the magnetic layer. If the magnetic layer contains too much lubricant, the physical properties thereof are deteriorated. Thus, the content of the lubricant in the magnetic layer is naturally limited.
If a ferromagnetic metal particle or hexagonal system ferrite in the form of ferromagnetic particles is incorporated in the magnetic layer to increase the capacity of magnetic recording discs, the magnetic layer tends to exhibit reduced running durability.
Further, the thinner the magnetic layer is, the more easily the magnetic layer can be peeled off from its support. Such peeling of the magnetic layer can be another cause of dropout. The peeling of the magnetic layer cannot be avoided even by providing a nonmagnetic layer between the magnetic layer and the nonmagnetic support.
As mentioned above, to provide a large capacity medium for a magnetic recording disc, such as floppy disc, the above problems must be considered. No means satisfying all these requirements have been proposed to date.